I managed to get it to work by adding the file to mappings in Universal. So for you, you would need something like this:

mappings in Universal += file("test.txt") -> "keys/test.txt"

You won’t need the COPY command if you do this, by the way.

Now, I’m not sure if this is going to add this mapping to other sbt-native-packager plugins. I hope a commenter can tell me whether or not this is true, but my intuition is that it will do so, which might be a dealbreaker for you. But any workaround is better than none, right? If you use Build.scala you could maybe use a VM argument to tell sbt whether or not to add this mapping…

You may place all additional files (which must be included in container image) into folder src/universal. Content of that folder will be automatically copied in /opt/app folder within your container image. You don’t need any additional configuration. See “Getting started with Universal Packaging” for additional info.